Fuel cells and air batteries are electrochemical energy devices in which energy generated by a chemical reaction of a compound serving as a fuel with a negative electrode active material is extracted as electric energy using oxygen in the air as an oxidant. Fuel cells and air batteries have a higher theoretical energy content than secondary batteries such as a Li ion battery, and can be utilized for in-vehicle power sources, stationary-type power sources in homes, factories, and the like, power sources for mobile electronic equipment, and the like.
An electrochemical reaction in which oxygen is reduced occurs at the oxygen electrode side in a fuel cell and an air battery. An oxygen reduction reaction hardly progresses at a relatively low temperature and can be generally promoted by a noble metal catalyst such as platinum (Pt). However, the energy conversion efficiencies of a fuel cell and an air battery have not been sufficient yet. In addition, since an oxygen reduction reaction occurs in a high potential region, it causes even a noble metal such as Pt to be dissolved and degraded, resulting in problems of long-term stability and securing of reliability. Furthermore, a catalyst containing a noble metal such as Pt as a main component is expensive, thereby driving up the costs of a fuel cell and an air battery as the whole system to prevent such batteries from becoming widespread. Accordingly, there is a demand for developing a catalyst that is inexpensive with no use of a noble metal such as platinum and that has a high oxygen reduction capacity.
As a catalyst not containing Pt, an organic metal complex, carbon nitride, a transition metal chalcogenide, a transition metal carbonitride, a transition metal nitride, and the like are known, but all of them are insufficient in terms of catalyst activity and durability, and do not achieve a higher performance than Pt-based catalysts.
Non Patent Literatures 1 and 2 disclose that some of oxides of transition metals, Group IV and V elements, inter alia, have an activity to an oxygen reduction reaction. In addition, Non Patent Literature 3 and Patent Literature 1 point out a possibility in which a structure defect partially serves as an active point of an oxygen reduction reaction. Furthermore, Non Patent Literatures 4 and 5, and Patent Literature 1 disclose that electron conductive carbon and the like are added at the time of making up an electrode.